Muhammad understands the Quran, again, he was just prohibited from telling it to the others, in fact he explicitly received the words of God directly, since he was a prophet anyway, how could he not understand it. If you're talking about some verses, even if no one except God understands them, the intention is to reveal their meaning eventually, the point being that they are still written into the Quran, so all of the details are fully in there, and the book is complete.
I'm not particularly impressed. Hey, no matter what I believe, I can call you out on being arrogant, since that goes against your religion.
The Bible and the Torah are edited texts.
There is, most people commit heresy by following the Hadiths for example, which is already a grave mistake, since they were written by Satan.
I'm not doing biddah. Get over it.